The invention relates to a screen for fiber suspensions, more specifically, to a screen of the type (so-called bar screen basket) that is the subject matter of WO 98/57723 of the company Hermann Finckh Maschinenfabrik GmbH and Co.
Such a cylindrical or possibly also conical screen of rotationally symmetrical shape in relation to a screen axis has several ring-shaped profiled bar supports (also referred to as supporting rings) arranged at spacings from one another in the direction of the screen axis and lying in planes extending perpendicularly to the screen axis, and these profiled bar supports are provided at their inner or outer circumference, mostly at the inner circumference, with a ring of cutouts arranged at spacings from one another in the circumferential direction and opening towards this circumference of the supports. Viewed in the direction of the screen axis, the cutouts of the ring-shaped profiled bar supports lie over one another so as to enable straight profiled bars to be inserted into these cutouts and held therein. These profiled bars extend transversely to the circumferential direction of the screen, are arranged at spacings from one another in the circumferential direction of the screen and form between them slot-shaped screen openings of identical width which are interrupted by the ring-shaped profiled bar supports. All the profiled bars usually have the same configuration (the spacings of the supporting ring cutouts from one another are then all of the same size) and have a cross section of elongate shape, with the longitudinal direction of the profiled bar cross section being at least approximately radially oriented in relation to the screen axis. However, profiled bars of different configuration may also be used and arranged at such spacings from one another that the slot-shaped screen openings nevertheless are all of the same width. With an end region of the profiled bar cross section each profiled bar is inserted into a respective cutout of each one of the profiled bar supports, and, viewed in the direction of the screen axis, the shape of these cutouts corresponds to the configuration of this cross section end region of the profiled bars and forms at a radial spacing from the open end of the cutout an undercut in which a projection of the profiled bar cross section engages so that the profiled bars are positively held in the cutouts of the profiled bar supports, both in a radial direction in relation to the screen axis and in the circumferential direction of the screen. In the manufacture of such a screen, the profiled bar supports provided with the cutouts may initially be present in the form of straight bars or rails which after insertion of the profiled bars into the cutouts are bent into closed supporting rings so as to thereby clamp the profiled bars in the supporting ring cutouts when the cutouts are located at that side of the profiled bar supports which later forms the inner circumference of the supporting rings. As described in WO 98/57723, the straight or ring-shaped profiled bar supports may, however, also undergo such plastic deformation on that side of the profiled bars facing away from the open ends of the cutouts that as a result of displacement of profiled bar support material in the direction towards the profiled bars, their projections are pressed against the undercuts formed by the cutouts, and the profiled bars are thereby fixed in the supporting rings-this measure can be taken in both initially ring-shaped profiled bar supports and initially straight profiled bar supports, and, in the latter case, before or after the profiled bar supports are bent into closed supporting rings.
In the manufacture of the known screens disclosed WO in 98/57723, a fundamental problem arises in connection with insertion of the profiled bars into the profiled bar supports, irrespective of whether the profiled bars are inserted into the cutouts of still straight profiled bar supports or into the cutouts of supporting rings: As the profiled bar support cutouts each form an undercut in which a projection of the profiled bar cross section engages, the profiled bars must either be pushed into the profiled bar support cutouts in the longitudinal direction of the profiled bars or snapped into these cutouts transversely to the longitudinal direction of the profiled bars. In the former case, the insertion of the relatively long and thin profiled bars involves considerable difficulties as the profiled bars are to be held with a snug fit in the profiled bar support cutouts, which results in a high slide resistance, and, in the latter case, when snapping the profiled bars into the profiled bar support cutouts, if not a plastic deformation, then at least a considerable elastic deformation occurs in the webs of the profiled bar supports located between the profiled bar support cutouts. Even if these webs of the profiled bar supports only undergo elastic deformation when snapping in the profiled bars and thereby widening the profiled bar support cutouts, upon inserting the last profiled bar into the profiled bar supports the webs of the profiled bar supports adjacent to this profiled bar are unable to deviate or deflect at least to any appreciable extent towards both sides, quite apart from the fact that the snapping of the other profiled bars into the profiled bar support cutouts requires considerable forces if the profiled bars are to be held in a clamped and snugly fitting manner in the profiled bar support cutouts in the finished screen. The problems described hereinabove conflict, above all, with automated assembly of the screens.
The object underlying the invention was to create a screen of the generic kind as described in WO 98/57723 or in any of the publications (EP-B-0 417 408 and EP-A-0 499 154) mentioned in this document, which is easier to assemble than these known screens (bar screen baskets) and which basically opens up the possibility of automated assembly of the profiled bars in or on the profiled bar supports.
This object is accomplished in accordance with the invention by a screen in accordance with claim 1 and by a manufacturing method in accordance with claim 20.
The invention thus relates to a screen for fiber suspensions having a shape which is essentially rotationally symmetrical in relation to a screen axis, with a first and a second circumferential side, one of which forms an inlet side and the other an outlet side of the screen for the fiber suspension, with a series of straight profiled bars arranged at spacings from one another in the circumferential direction of the screen, extending transversely to the circumferential direction of the screen and forming slot-shaped screen openings of identical width between them, the cross sections of the profiled bars each having an elongate shape with a first end region facing away from the second circumferential side of the screen and a second end region facing away from the first circumferential side of the screen, and with several ring-shaped profiled bar supports arranged at spacings from one another in the direction of the screen axis and lying in planes extending perpendicularly to the screen axis, each of the profiled bar supports having in its first edge region facing away from the second circumferential side of the screen a series of cutouts opening towards the first circumferential side of the screen, the shape of the cutouts-viewed in the direction of the screen axis-corresponding to the configuration of the second cross-sectional end regions of the profiled bars lying in these cutouts, and forming at a radial spacing from the first edge of the profiled bar support facing the first circumferential side of the screen an undercut in which a projection of the profiled bar cross section engages so that the profiled bars are held positively in a radial direction in relation to the screen axis and in circumferential direction of the screen with their second cross-sectional end regions in the profiled bar support cutouts, with the profiled bars preferably projecting with their first cross-sectional end regions in a radial direction over the first edges of the profiled bar supports. In accordance with the invention it is proposed that such a screen be configured such that the second cross-sectional end region of each profiled bar has for each profiled bar support a recess which decreases the width of the second cross-sectional end region measured in the circumferential direction of the screen, and the recesses of each profiled bar are arranged at spacings from one another corresponding to the spacings of the profiled bar supports and are located outside the profiled bar supports in the finished screen, and that the profiled bar recesses are of such configuration that the areas of the second cross-sectional end regions of the profiled bars provided with these recesses are insertable transversely to the longitudinal direction of the pertinent profiled bar into the profiled bar support cutouts at least without any substantial plastic deformation of the profiled bar supports and/or the profiled bars. Outside their recesses the profiled bars preferably have the same cross section over all (in shape and size). Furthermore, embodiments are preferred which all the profiled bars are of identical design.
To manufacture such a screen, it is proposed in accordance with the invention that the profiled bars be inserted with their areas provided with the recesses into the cutouts of the profiled bar supports in a direction perpendicular to the longitudinal direction of the profiled bars, and the profiled bar supports and the profiled bars then be displaced relative to one another in the longitudinal direction of the profiled bars such that the profiled bar recesses lie outside the profiled bar supports and the second cross-sectional end regions of the profiled bars lie at least substantially free of play in the profiled bar support cutouts.
The invention makes it possible to push the profiled bars into the cutouts of the profiled bar supports transversely to their longitudinal direction at least almost without resistance, whereupon the profiled bars and the profiled bar supports only have to be displaced slightly relative to one another in the longitudinal direction of the profiled bars in order to secure the profiled bars in the profiled bar support cutouts. As both procedures can be carried out by relatively slight movements of the profiled bars and/or the profiled bar supports, without having to overcome any appreciable resistances, the invention also makes it possible to automate assembly of a screen according to the invention without having to fear that the relatively long and thin profiled bars and/or the profiled bar supports will be bent.
In so-called pressure sorters for fiber suspensions comprising a rotationally symmetrical, basket-like screen (albeit open at the top and bottom) and a rotor which rotates about the screen axis and is either arranged within the screen basket on its outer side (by comprising, for example, blade-like elements which run past the outer side of the screen basket), the screen is subjected to considerable pressure loads. These pressure loads are due, firstly, to the fiber suspension to be treated being supplied to the pressure sorter under, and, in addition, to positive and negative pressure pulses being generated in the fiber suspensions, adjacent to the circumferential wall of the screen, by elements provided on the rotor, and the higher the so-called material density of the fiber suspension (material density is understood as the solids content of the fiber suspension, which is formed by fibers and impurities contained in the fiber suspension), the stronger are these alternating pressure loads on the screen wall. The profiled bars between the profiled bar supports are subjected to bending by these pressure loads. It has, however, been found that the necessary resistance of the profiled bars to these pressure loads is virtually unimpeded by the recesses to be provided on the profiled bars in accordance with the invention, if in the finished screen profiled bar recesses lie in the vicinity of the profiled bar supports. In preferred embodiments of the screen according to the invention, all the profiled bar recesses are arranged immediately adjacent to the profiled bar supports because the weakening of the profiled bars by their recesses with respect to the resistance of the screen to the pressure loads described hereinabove is insignificant.
In order that the profiled bars will be insertable into the profiled bar support cutouts at least substantially without resistance, it is recommended that the length of the profiled bar recesses measured in the longitudinal direction of the profiled bars be the same as or slightly larger than the thickness of the profiled bar supports measured in the direction of the screen axis.
In order to avoid the occurrence of an undesired notch effect on the profiled barsxe2x80x94such notch effects increase the risk of breakage of the screen under the influence of the pressure loads described hereinabovexe2x80x94preferred embodiments of the screen according to the invention are characterized in that each profiled bar recess (in a side view of the pertinent profiled bar) passes over without any edges into the profiled bar areas adjacent to the recess in the longitudinal direction of the profiled bar. In particular, a concave recess end region adjoins the actual profiled bar recess (in the longitudinal direction of the profiled bar) on both sides so as to ensure a smooth and continuous transition of the recess into the adjacent, unweakened areas of the profiled bar. In such embodiments, it will be advantageous to make the length of the actual profiled bar recess measured in the longitudinal direction of the profiled bar (without the recess end regions mentioned hereinabove) the same size as the thickness of the profiled bar supports measured in the direction of the screen axis.
If in the manufacture of the screen according to the invention, the profiled bars are inserted into cutouts of straight profiled bar supports and the latter are then bent into circular supporting rings closed within themselves so that the cutouts of the profiled bar supports are located at the inner circumference of the supporting rings, the internal tension of the profiled bars in the profiled bar support cutouts which accompanies the bending may be adequate to secure the profiled bars against longitudinal displacements relative to the profiled bar supports. If the profiled bars are inserted into cutouts of profiled bar supports which are already ring-shaped, various measures could be taken to secure the profiled bars against longitudinal displacement. For example, stop rings for the profiled bars could be provided at both axial ends of the screen, and these and also the profiled bar supports could be joined to one another by, for example, welded-on bars extending in the direction of the screen axis. However, a different securing measure, which is the subject matter of the above-mentioned WO 98/57723 of the company Hermann Finckh Maschinenfabrik GmbH and Co is preferred, according to which preferred embodiments of the screen according to the invention are characterized in that the profiled bar supports have such deformed areas at the side of the profiled bars facing the second circumferential side of the screen that as a result of a displacement of profiled bar support material in the direction towards the first circumferential side of the screen, the profiled bar projections are pressed in this direction against the undercuts formed by the profiled bar cutouts.
Further improvements of this securing measure will be apparent from WO 98/57723. Mention is also made of the fact that the securing of the profiled bars against longitudinal displacements by deformation of the profiled bar supports can be carried out not only on ring-shaped profiled bar supports but also on still straight, profiled bar supports which already hold the profiled bars and are subsequently shaped into supporting rings.
Further improvements of the screen according to the invention and of the method according to the invention for manufacture thereof will be apparent from the attached claims and the appended drawings as well as the following description of a preferred embodiment of the screen according to the invention.